1. Field of the Invention
The present invention relates to a method and apparatus for controlling inter-Public Land Mobile Network (PLMN) handover to a Closed Subscriber Group (CSG) cell. More particularly, the present invention relates to a method and apparatus for controlling inter-PLMN handover based on CSG lists of equivalent PLMNs.
2. Description of the Related Art
A Closed Subscriber Group (CSG) is a limited set of users permitted to access a specific cell (i.e., a CSG cell assigned a CSG ID). In other words, only those users included in the CSG list are permitted to access the specific cell. For example, when a femtocell is deployed for use in a specific location, such as a user's home, it can be configured in CSG mode in which it is assigned a CSG ID. In that case, the CSG ID is added to the allowed CSG list of the family members' terminals so as to allow access to the femto cell.
The control nodes of a network including a Home evolved Node B (HeNB) that are assigned a CSG ID can be configured as shown in FIGS. 1 and 3.
FIG. 1 is a diagram illustrating a configuration of network control nodes including an HeNB having a CSG ID according to the related art. FIG. 3 is a diagram illustrating another configuration of network control nodes having a CSG ID according to the related art.
Referring to FIG. 1, macro eNB1 17 and macro eNB2 20 having no CSG ID and HeNBs 15, 16, 18, and 19 having a CSG ID are respectively connected to a Mobility Management Entity (MME) 11 and 12, which are in turn each connected to Home Subscriber Server (HSS) 10. The connection to the MME can be achieved via an HeNB Gateway (GW) 13 and 14, as used by the HeNBs 15, 16, and 18, or without involvement of an HeNB GW as in the case of the HeNB 19. Because the fact of whether the HeNB is connected to the MME through the HeNB GW does not influence the procedure related to the present invention, a detailed description thereof is omitted
Similarly, referring to FIG. 3, both macro eNBs 37 and 40 having no CSG ID and HeNBs 35, 36, 38 and 39 having a CSG ID are connected to an MME 31 and 32, respectively, which are in turn each connected to HSS 30. The connection to an MME can be achieved via an HeNB GW 33 and 34, as used by the HeNBs 35, 36, and 38, or without involvement of an HeNB GW as in the case of the HeNB 39. Because the fact of whether the HeNB is connected to the MME through the HeNB GW does not influence the procedure related to the present invention, a detailed description thereof is omitted.
Problems that may arise in the networks configured as shown in FIGS. 1 and 3 are described with reference to FIGS. 2 and 4, respectively.
FIG. 2 is a signaling diagram illustrating a handover procedure in the network configured as shown in FIG. 1 according to the related art.
Referring to FIG. 2, User Equipment (UE) 21 is configured with the allowed CSG lists of {(PLMN A, (CSG1, CSG2)), (PLMN B, (CSG1, CSG3))} at step 100 which is the same CSG list of the HSS 10. Here, PLMN A and PLMN B are equivalent PLMNs. In this state, if the UE 21 sends an ATTACH or a TRACKING AREA UPDATE (TAU) request to the MME1 11 of the PLMN A at step 101, the MME1 11 processes the ATTACH or the TAU request and sends a Location Update Request to the HSS 10 so as to register itself as the serving MME of the corresponding UE 21 and acquire the subscription information of the UE 21 at step 101.a. The subscription information includes the CSG list of the PLMN A as the registered PLMN and is delivered to the MME1 11 at step 101.b. After the registration process has completed, the CSG list of the PLMN B is updated. In this case, the CSG1 may be unintentionally deleted in the CSG list at step 103. Such a CSG list update situation of the HSS 10 is delivered through Open Mobile Alliance (OMA) Device Management (DM). However, since the allowed CSG list update execution timing in the UE 21 using the OMA-DM is not synchronized with the HSS 10 update timing accurately, the allowed CSG list of the UE 21 is in the un-synchronized state for a certain time period until the OMA-MD is executed, even though the allowed CSG list of the HSS 10 has been updated. In such a situation, the UE 21 may approach the HeNB3 18 at step 102. If the UE 21 moves towards the HeNB3 18 in the state that in which its allowed CSG list is un-synchronized, the measurement report is performed based on the old allowed CSG list which has not been updated yet. That is, the UE 21 determines whether the CSG ID of the HeNB3 18 exists in the PLMN's allowed CSG list stored in the UE 21. Since the CSG ID of the HeNB3 18 is CSG1 and the PLMN B's CSG list of the UE 21 includes the CSG1, the determination result is true such that the UE 21 reports the CS cell (i.e., the HeNB3's cell) to the eNB1 17 as the current serving cell at step 104. If the report is received, the eNB1 17 evaluates the information on the equivalent PLMN and the CSG membership reported by the UE 21 to determine inter-PLMN handover to the target CSG cell of a different PLMN, resulting in a handover decision for the UE 21 to the CSG1 of the PLMN B and sending a Handover Required message to the MME1 11 (PLMN A) at step 105. If the Handover Required message is received, the MME1 11 determines handover based on the allowed CSG list of the PLMN A which is received from the HSS 10 at step 101.b other than the allowed CSG list of the target PLMN B. Since the CSG1 as the CSG ID of the target cell exists in the allowed CSG list of the PLMN A, the MME1 11 determines that the UE 21 has access rights to the CSG1 and, as a consequence, determines that the PLMN B is the equivalent PLMN of the PLMN A. In that case, the MME1 11 executes the handover by sending a forward relocation required message to the MME2 12 as the target MME at step 106. Since the MME2 12, as the target MME, is the equivalent PLMN of the source PLMN, it performs the handover preparation procedure at step 107 and the handover execution procedure entirely such that the UE 21 completes the handover to the HeNB3 18 at step 108. Since the TA is changed due to the change of the PLMN after the completion of the handover, the UE 21 sends a TAU message to the MME2 12 as the new MME at step 109. If the TAU message is received, the MME2 12 sends a location update request to the HSS 10 at step 110 for the TAU to register the MME2 12 as the serving MME of the UE 21 and receives the subscription data including the PLMN B's allowed subscription list of CSG3 at step 111. At this time, the MME2 12 determines that the UE 21 is not authorized for CSG1 in step 112. That is, the received PLMN B's allowed CSG list does not include CSG1. Accordingly, the TAU request of the UE 21 is rejected at step 113 and the UE 21 enters the unregistered state. Thus, if the procedure depicted in FIG. 2 is executed in the network configured as shown in FIG. 1, a problem is caused in that the UE 21 completes the handover and then loses network connection.
FIG. 4 is a signaling diagram illustrating a handover procedure in the network configured as shown in FIG. 3.
Referring to FIG. 4, UE 41 is configured with the allowed CSG lists of {(PLMN A, (CSG1, CSG2)), (PLMN B, (CSG2, CSG3))} in step 200 which is the same CSG list of the HSS 30. Here, PLMN A and PLMN B are equivalent PLMNs. In this state, if the UE 41 sends an ATTACH or a TAU request to the MME1 31 of the PLMN A at step 201, the MME1 31 processes the ATTACH or TAU request and sends a Location Update Request to the HSS 30 at step 201.a so as to register itself as the serving MME of the corresponding UE 41 and acquire the subscription information of the UE 41 at step 201.b. The subscription information includes the CSG list of the PLMN A as the registered PLMN and is delivered to the MME1 31. In this situation, the UE may approach the HeNB4 38 at step 202. In that case, the UE 41 performs a measurement report based on its allowed CSG list. That is, the UE 41 determines whether the CSG ID of the HeNB4 38 exists in the corresponding PLMN's allowed CSG list stored in the UE 41. Since the CSG ID of the HeNB4 38 is CSG3 and the CSG3 exists in the corresponding PLMN's CSG list of the UE 41, the determination result is true such that the UE 41 reports the CSG3 as a cell of the HeNB4 38 to the eNB1 37 as the current serving cell at step 204. If the report is received, the eNB1 37 makes an inter-PLMN handover decision to a CSG cell of another PLMN using the membership information and equivalent PLMN information reported by the UE 41 and sends a Handover Required message to the MME1 31 (PLMN A) at step 205. If the Handover Required message is received, the MME1 31 makes a handover decision at step 206 based on the allowed CSG list of the PLMN A which is received from the HSS 30 at step 201.b other than the allowed CSG list of the target PLMN B. If the CSG3 as the CSG ID of the target cell does not exist in the allowed CSG list of the PLMN A, it is determined that the UE 41 has no access right to the CSG3 such that the handover is rejected at step 207, even though the UE can actually make a handover to the CSG 3. As a result, although it is possible to be served by the HeNB4 38 having a good channel condition, the UE 41 is being served by the eNB1 37 having a poor channel condition.
That is, the procedure shown in FIG. 4 that is executed in the network configuration of FIG. 3 causes a problem in that the UE fails to perform a handover to the best cell available, resulting in a potential failure of the communication.